Sodium chloride tends to form large, agglomerated masses upon exposure to moisture, particularly during long periods of storage. These hardened masses are generally referred to as cakes. A non-caking agent is often added to the salt to prevent the formation of cakes. In recent years much effort has been put into the development of improved non-caking salt agents which are inexpensive and environmentally safe, and which are effective in small amounts. The iron complex of a mixture of tartaric acids comprising meso-tartaric acid was found to be an effective non-caking additive for sodium chloride. Particularly preferred is a non-caking additive comprising an iron complex of a mixture of tartaric acids, with between 55 and 90% by weight, more preferably with between 60 and 80% by weight, thereof being meso-tartaric acid.
Several stereoselective synthetic routes towards pure meso-tartaric acid exist. However, these methods are either not economically attractive or undesired byproducts are formed. For instance, it was found that epoxidation of fumaric acid with concentrated H2O2 followed by hydrolysis leads to formation of only the meso-isomer of tartaric acid, without the use of any metal salts. However, relatively harsh process conditions, low conversion and byproduct formation make this route not very attractive. Furthermore, it has been found that maleic acid can be converted into meso-tartaric acid in presence of KMnO4. The main drawback of this route is the stoichiometric consumption of KMnO4 and the need to separate the meso-tartaric acid out of the meso-tartaric acid manganese salt—for application as non-caking additive on sodium chloride the meso-tartaric acid has to be virtually Mn-free. Along the same lines, Mn/Amine complex as catalyst or oxidizing agent and optionally H2O2 may be used to convert maleic acid into meso-tartaric acid, but such routes have similar product purification challenges.
WO 00/59828 discloses in the Examples a method for producing a mixture of tartaric acids which includes meso-tartaric acid. It mentions that it can be prepared by treating a natural or synthetic tartaric acid (CAS registry numbers 87-69-4 and 147-71-7, respectively) solution with concentrated NaOH at temperatures above 100° C. Part of the L-, D- and/or DL-tartaric acid is then converted to the desired meso-tartaric acid (CAS registry number 147-73-9). However, it was found that by following this procedure, it is merely possible to prepare tartaric acid mixtures with up to a maximum of 50% by weight of the tartaric acid being the meso isomer. Until now, however, no easy and economically attractive processes existed for the preparation of mixtures of tartaric acid comprising over 50% by weight of meso-tartartic acid.